Hemoglobin-based oxygen carriers (HBOCs) are candidates for use as a blood substitute and resuscitation fluid. We determine that chemical modification used to generate hemoglobins crosslinked at either the alpha or beta chains alter their ability to generate or interact with free radicals. Exposure of crosslinked hemoglobins to enzymatically generated hydrogen peroxide (Xanthine/xanthine oxidase system) causes generation of hydroxyl radicals (OH), capable of cleaving dimethyl sulfoxide to form formaldehyde. Our results indicate that HBOCs exhibit a significant difference from each other and from unmodified hemoglobins in reactions involving the production of (OH). The relative ability of the ferric derivatives of these HBOCs to participate in free radical reactions was monitored by nonenzymatic NADPH oxidation and aniline hydroxylation assays (reactions mediated by reactive oxygen species). Crosslinked hemoglobins again exhibited significant differences in their reactivity with oxygen species such as peroxides and superoxides. A number of in vitro and in vivo studies are planned in order to continue to examine the hemoglobin-mediated radical generation and to determine how these radicals may aggregate "reperfusion injury" in an ischemic animal model. Results of some of these studies were presented at the IV International Symposium on Blood Substitutes (August 19-23 1991) in Canada. A manuscript describing this work in full is being prepared for publication.